The present invention relates to a method for the preparation of 4,6-dichloropyrimidine from 4-chloro-6-methoxypyrimidine. 4,6-Dichloropyrimidine is a valuable intermediate for the preparation of crop protection agents.
A number of methods for preparing 4,6-dichloropyrimidine are known starting from 4,6-dihydroxypyrimidine.
It is also known (see Res. Discl. n 391, 690-691 (1996)) that 4,6-dichloropyrimidine can be reacted by reacting 4-chloro-6-methoxypyrimidine with a chlorinating agent of the formula R3PCl2. The chlorinating agent can be employed as such or be prepared in situ from a compound of the formula R3Pxe2x95x90O and phosgene. It is additionally described therein that 4-chloro-6-methoxypyrimidine does not react with phosphorus oxychloride. The disadvantage of this method is that usually only very incomplete conversion can be achieved and thus 4,6-dichloropyrimidine is obtainable only in low yields and low degrees of purity.
A method for the preparation of 4,6-dichloropyrimidine from 4-chloro-6-methoxypyrimidine has now been found and is characterized in that 4-chloro-6-methoxypyrimidine is reacted with at least one sulphur-containing chlorinating agent selected from the group of SCl2, SOCl2 and SO2Cl2 and in the presence of at least one phosphorus compound of the formula
R3Pxe2x95x90Ynxe2x80x83xe2x80x83(1),
in which
R represents C1-C10-alkyl or C6-C10-aryl, each of which may optionally be substituted by up to 5 identical or different substituents from the group of fluorine, chlorine, bromine, C1-C4-alkyl and C1-C4-alkoxy,
Y represents oxygen or sulphur
and
n represents zero or 1.
Preferred sulphur-containing chlorinating agents are SOCl2 and SO2Cl2, and preferred phosphorus compounds are triphenylphosphine and triphenylphosphine oxide.
Based on 1 mol of 4-chloro-6-methoxypyrimidine it is possible, for example, to employ at least 1 mol of sulphur-containing chlorinating agent. This amount is preferably 1 to 2 mol.
Based on 1 mol of sulphur-containing chlorinating agent it is possible to employ, for example, 0.01 to 1 mol of phosphorus compounds of the formula (1). This amount is preferably 0.02 to 0.25, particularly preferably 0.05 to 0.1, mol.
The method according to the invention is preferably carried out in the presence of a solvent. Suitable examples are aromatic solvents such as toluene, xylenes, chlorobenzene, dichlorobenzenes, chlorotoluenes, benzonitrile and benzotrifluoride, nitrogen-containing solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide and cyclic ureas and oxygen-containing solvents such as ethers, in particular higher-boiling ethers and polyethers. It is also possible to employ mixtures of solvents.
The method according to the invention can be carried out, for example, at temperatures in the range from 50 to 200xc2x0 C. 75 to 175xc2x0 C. are preferred, especially 100 to 150xc2x0 C.
The pressure is in principle not critical. Only if it is wished to employ solvents which boil below the desired reaction temperature at atmospheric pressure is it indicated to operate under elevated pressure so that the solvent is present at least partly in liquid form.
On the assumption that, at the given temperature, no precursors, products and solvents escape from the reaction vessel, the pressure can be, for example, in the range from 0.1 to 5 bar. The pressure is preferably atmospheric, particularly preferably using a solvent which boils under reflux at the required reaction temperature under atmospheric pressure.
The method of the invention can be carried out batchwise and continuously.
In a preferred embodiment of the method according to the invention, 4-chloro-6-methoxypyrimidine, the phosphorus compound and a solvent are mixed, the mixture is heated to the reaction temperature, and then the sulphur-containing chlorinating agent is metered in, where appropriate in a plurality of portions.
The reaction mixture present after the reaction can be worked up, for example, by distilling it through a column, where appropriate under reduced pressure.
It is possible with the method according to the invention to convert 4-chloro-6-methoxypyrimidine into 4,6-dichloropyrimidine in a simple manner. The conversion takes place virtually completely, in contrast to the prior art. This makes it possible to prepare a product with a high 4,6-dichloropyrimidine content in a simple manner. This is desirable because unreacted 4-chloro-6-methoxypyrimidine can be removed only with difficulty by distillation.